1. Field of the Invention
The present invention relates to a wavelength variable light source in which a plurality of semiconductor lasers is integrated, and a wavelength variable light source module including the wavelength variable light source.
2. Description of the Background Art
In recent years, a wavelength division multiplexing communication system that enables a large capacity transmission with one optical fiber by multiplexing a plurality of signal light beams having different wavelengths is being realized with significant increase in the demand for communication.
An LD (Laser Diode) of single mode (hereinafter referred to as single mode LD) in which high side mode suppression ratio (SMSR) of at least greater than or equal to 30 dB to 40 dB is obtained is suitable for a light source used in the wavelength division multiplexing communication system. The single mode LD includes, for example, a distributed feedback LD (hereinafter referred to as DFB-LD), a distributed Bragg reflector laser diode (hereinafter referred to DBR-LD), and the like.
In order to realize the wavelength division multiplexing communication system, a low-cost wavelength variable light source that covers the entire wavelength band is necessary. An LD light source monolithically integrated on the same substrate has been given attention for the wavelength variable light source.
The wavelength variable light source is generally used in combination with an external modulator module that generates a data signal, but in addition, researches on monolithically integrating an electro-absorption (EA) light modulator and a Mach Zehnder (MZ) light modulator on the same substrate are also being carried out.
Conventionally, a wavelength variable light source configured so that an output side of a plurality of DFB-LDs is connected to an input waveguide of a multi-mode interference type optical multiplexing circuit (hereinafter referred to as MMI), and the light multiplexed by the MMI is output from an output waveguide is disclosed (see e.g., Japanese Patent No. 3887744). Hereinafter, the MMI of K input (K is a natural number) and L output (L is a natural number) is denoted as K×L-MMI. In other words, the K input refers to the K input waveguides connected to the MMI, and the L output refers to the L output waveguides connected to the MMI.
A wavelength variable light source in which an output side of a plurality of Ins is connected to N×2-MMI (N is a natural number of greater than or equal to three) and two output waveguides of the MMI are connected to two arms of the Mach Zehnder modulator is disclosed (see e.g., Japanese Patent No. 4728746). According to Japanese Patent No. 4728746, the light loss can be reduced compared to the case where the N×1-MMI is used.
A wavelength variable light source configured so that an output side of a plurality of DFB-LDs is connected to the MMI, and phase adjustment is performed at each of the two output waveguides of the MMI and then the light is output from one output waveguide through the other MMI is disclosed (see e.g., Japanese Patent Application Laid-Open No. 2011-44581). According to Japanese Patent Application Laid-Open No. 2011-44581, higher output can be realized and SN ratio (Signal to Noise ratio) can be improved.
In a wavelength division multiplexing communication system in which the transmission speed is greater than or equal to 40 Gbps in a mainline system, digital coherent communication using an optical phase modulation has been put into practical use in recent years. In the wavelength division multiplexing communication system adopting a digital coherent communication, a wavelength variable light source of a narrow line width having a laser oscillation line width of smaller than or equal to 1 MHz, and more desirably smaller than or equal to 500 kHz is used as the light source for transmission/reception.
In the wavelength division multiplexing communication adopting the digital coherent communication, a separate wavelength variable light source is required for transmission and for reception when the wavelengths of the light signals used for transmission and for reception are different from each other, which increases the power consumption of the entire transmission/reception device equipped with the wavelength variable light source.
In the wavelength division multiplexing communication adopting the digital coherent communication, the wavelengths of the light signals used for the transmission and for the reception are the same in some cases, and generally, the light output of the wavelength variable light source of a narrow line width that is required for the transmission and for the reception is different. In such a case, one wavelength variable light source is desirably provided to be used for the light source for transmission and for reception from the standpoint of reducing the power consumption and reducing the mounting area of the entire transmission/reception device.
For example, in a case in which the wavelength variable light source of each of Japanese Patent No. 3887744, Japanese Patent No. 4728746, and Japanese Patent Application Laid-Open No. 2011-44581 is used for the light source for transmission and for reception, the wavelength variable light source of each of Japanese Patent No. 3887744, Japanese Patent No. 4728746, and Japanese Patent Application Laid-Open No. 2011-44581, which is one-input, needs to be made to two-outputs with a polarization maintaining coupler. However, branching loss occurs in the polarization maintaining coupler in such a configuration, and thus an injecting current with respect to a semiconductor optical amplifier (hereinafter referred to as SOA) arranged on the output side of the wavelength variable light source needs to be increased to increase the light output from the wavelength variable light source in order to compensate for the branching loss. This may become the cause of increase in the power consumption and the laser oscillation line width.